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PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



[Feb. 6, 



other part of the district, and none certainly which showed such a 

 regularly stratified arrangement. One of us, accordingly, ascended 

 the craggy sides of Scuir Dhu, and thence obliquely southwards to 

 the top of the highest of the Beann Taobhliath hills, or "Grey 

 Heads." The following section represents, better than any detailed 

 description, the structure of Scuir Dhu. 



Fig. 10. — Section of Scuir Dhu. 



c 



b. Cambrian sandstone and conglomerate. c. Quartz-rock. 



The Cambrian at the base of the hill dips E. 10° to 20° S., at 30° 

 to 40°. It is covered by white quartzite having the same dip and 

 angle, and showing still the oblique lamination or " false-bedding " 

 of the original sand. Did no other section exist, we should say that 

 the Silurian rocks here followed the Cambrian in conformable se- 

 quence. The Cambrian beds slope up the valley along the lower part 

 of the acclivity, while the quartz-rock rises above them to form the 

 greater part of the north-eastern side of the glen. The latter rock is 

 occasionally deeply channeled by mountain-torrents. One ravine 

 in particular occurred in the course of the ascent, about 80 or 100 

 feet, and, in places, so narrow that it seemed as though one could 

 almost have stepped across it. Cambrian blocks strew the hillside, 

 and are often perched on the edges of cliffs and on the summits of 

 detached crags. None of them are of great size, the largest observed 

 being eight feet long, six broad, and three high. Gneiss-boulders 

 also occur, but are small in size as well as few in number. The 

 quantity of Cambrian blocks increases as we ascend, until, not far 

 below the summit, we come upon a mass of Cambrian sandstone in 

 place, dipping E. 4° S., at 27°. Between the quartz-rock and this 

 Cambrian ledge there is undoubtedly a fault, the former rock having 

 its beds curved and thrown back against the latter : this is shown 

 in the subjoined section (fig. 11), which is an enlarged view of that 

 part of fig. 10, marked *, The Cambrian beds are then covered 

 over by flat undulating beds of quartz-rock, which lie at the base of 

 a cliff of similar Cambrian sandstones. At the top of this cliff a small 

 ledge of quartz -rock exists (as shown in fig. 10), let in by a fault 



